I've got my hands on a defect c-64. It will start with borders ok and a screen full of scrambled characters with usually on or more "flickering" characters.
Now the fun thing is when I insert a power cartridge, freezing usually succeeds with a correct power cartridge 'freeze' screen, except for one thing.. The logo consists of 4 sprites, and sprite 3 and 4 are the same sprite 4! (So you'll see the "starry" 1985 part duplicated twice).
More fun is I can actually start the power cartridge monitor roughly one out of 4 tries. I filled memory from 1000-9FFF with AA's and 55's, saved that to disk and tried to analyze what is going on in RAM. A saved romdump of the kernel shows no troubles so ROM access seems to be ok..
Ram however is another thing.. I found errors in bits 0, 1, 5 and 6 all over the place and there seem to be some patterns.. For starters, its always only one bit failing at a certain location.
Bit 0 fails on most $XXFA, $XX87 and $XX92 locations.
Bit 1 fails on most $XXE8 locations.
Bit 5 fails on most $XX95 and $XXA2 locations.
Bit 6 fails on most $XX89 locations (and $XX2D, $XX13, $XXA1 more seldom).
Always only those addresses, those bits. Other addresses show no problems.
My very first guess before analyzing the memory dump was a faulty PLA, but this pattern failures made me think it might be multiplexers U13 or U25. But I can't make any sense in combining the failures to pinpoint the defect chip
So if some experts are around.. What would be more probable? A faulty CASRAM signal from the PLA? 4 defect RAM chips? One of the multiplexers? Or something I overlooked?
Like to have my theory of what is wrong in place before I start soldering parts in and out